Maybe speak to the son this timeand explain how hard it is for you to get in and out of your drive when he parks the way he does.. and if he still does not listen you have every right to get arsey. It happens to us. Maybe stick a polite notice on his van!!!!!
if you really dont want to risk any form of confrontation or funnyness over this, you could just go out in the car several times a day (dont know how convenient it would be!!) and keep going to their door saying "i'm so sorry to bother you but please move your van for a sec because i need to get my car out/park my car"
dont ASK for the van to be moved, just politely say move the van for a minute.
pop out at 7am a couple of times (if you can!) and maybe go out, come back and go out again half an hour later! just keep this sort of thing up for a bit and each time just again say please move the van for a sec cos i cant get out with it there.
the idea is to make keep moving the van a pain in the arse for him! that way, he will get sick of keep moving it for you and the idea will come from HIM to not park it there anymore, so he has no cause for being off about it!
but if it were me, i would just say oy, arsewipe, dont park your rustbucket here again or you'll turn up in a&e with it halfway up your bum.
I will take some advice from this thread, people always park opposite our drive, they do park half on the path but we have a mpv its quite long and we need to touch the kerb opposite our drive in order to get out.
Once I watched out for the person who regularly parks there and waited for her to park, i reversed my car before she got out to show her what difficulty i have getting out iykwim, this solved the problem for about two weeks but this week she has started to park there again arghhh